My experience is that they are nigh on impossible to clean! I have tried detergent, simple green, plain soap and murphys on mine- didn't have ANY effect at all getting the mud stains off it. Might lessen the diesel stains, but not likely that it will ever get clean. I would be real careful with a pressure washer- especially on the roof as it might not be waterproof afterwards; canvas relies on the swelling of the fibers to make the truly waterproof. Mine is over 10 years old now and has never leaked a drop EXCEPT where there is a hole or if I touched a spot and induced leakage.
Good luck!
